ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-8-5423-2008Inhibition of ice crystallisation in highly viscous aqueous organic acid dropletsMurrayB. J.11School of Chemistry, Woodhouse Lane, University of Leeds, Leeds LS2 9JT, UK1009200881754235433This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from http://www.atmos-chem-phys.net/8/5423/2008/acp-8-5423-2008.htmlThe full text article is available as a PDF file from http://www.atmos-chem-phys.net/8/5423/2008/acp-8-5423-2008.pdf

Homogeneous nucleation of ice within aqueous solution droplets and their
subsequent crystallisation is thought to play a significant role in upper
tropospheric ice cloud formation. It is normally assumed that homogeneous
nucleation will take place at a threshold supersaturation, irrespective of
the identity of the solute, and that rapid growth of ice particles will
follow immediately after nucleation. However, it is shown here through
laboratory experiments that droplets may not readily freeze in the very cold
tropical tropopause layer (TTL, typical temperatures of 186–200 K). In these
experiments ice crystal growth in citric acid solution droplets did not
occur when ice nucleated below 197&plusmn;6 K. Citric acid,
2-hydroxypropane-1,2,3-tricarboxyllic acid, is a molecule with similar
functionality to oxygenated organic compounds which are ubiquitous in
atmospheric aerosol. It is therefore thought to be a sensible proxy for
atmospheric organic material. Evidence is presented that suggests citric acid
solution droplets become ultra-viscous and form glassy solids under
atmospherically relevant conditions. Diffusion of liquid water molecules to
ice nuclei is expected to be very slow in ultra-viscous solution droplets
and nucleation is negligible in glassy droplets; this most likely provides
an explanation for the experimentally observed inhibition of ice
crystallisation. The implications of ultra-viscous and glassy solution
droplets for ice cloud formation and supersaturations in the TTL are
discussed.